You might as well try the fittings that come with trailer and decide on a fix if they are not up to the job.
I made a hitch with some ally offcuts, armoured cable cleat round chainstay and pick up on the skewer. It’s a single sided trailer hitch so would foul the derailleur on yours. Armoured cable cleats are good for sharing the load on to seat or chainstays if you do need to make something up. I think sizes 5, 6 or 7 are the most useful, roughly equates to 1/2″ 5/16″ and 3/4″